Method of softening hard water with sodium phosphate glasses



United States Patent Ofifice 3,130,152 Patented Apr. 21, 1954 3,15%,152hiETilfil) F SQFTENEIG Wl'lll Robert J. Fuchs, Clmh, l ll, assignor toFMQ Corporation, New York, FLY a corporation of Delaware No DrawinGriginal application @et. 5, 1961, Ser. No. 143,956. Bivitled and thisapplication May 29, 1963, Ser. No. 284,996

1 Claim. (Cl. 216-57) The present invention relates to a novel class ofsodium polyphosphate glasses having metal-corrosion inhibitingproperties when applied in aqueous solutions and more particularly, to aclass of sodium polyphosphate glasses which have improved stabilityagainst reversion to orthophosphates.

This application is a divisional of Serial No. 143,656, filed October 5,1961 in the name of Robert 3. Fuchs, entitled Sodium Phosphate Glasses.

Polyphosphate glasses are currently used to treat water because of theirability to inhibit corrosion of metals in contact with aqueoussolutions. Polyphcsphate glasses are produced by condensing molecules ofsodium orthophosphate to form longchained molecules having POP bonds.The condensation is carried out by driving oil molecular water at hightemperatures from the appropriate orthophosphate salts. The averagenumber of sodium orthophosphate molecules which have been condensed intoone polyphosphate molecule is expressed as the mole ratio of (Na O 2 5mole ratios of 1.33 to 1.14. Those having ratios above 1.2 have gooddispersing properties and are generally used in food applications.However, they are not normally suitable in water treatment. The commoncommercial phosphate glasses used in water treatment contain about 67 to67.5% P 0 and have mole ratios of 1.2 to 1.14. These polyphosphateglasses have the following formulation:

i M O? 0 Na n where 11:10 to 14 and M=Na or H, with Na being present inmajor molar amounts.

These polyphosphate glasses are commonly used in aqueous solutions toinhibit corrosion of metals in contact with aqueous solutions or toinhibit hard water scale and to simultaneously soften the water. Thislatter function is carried out by sequestering the metal ions present inan aqueous system in the form of soluble complexes of the offendingions, thereby preventing them from deleteriously afiecting the aqueoussystem.

One of the problems that has arisen with the use of glassypolyphosphates is the reversion of these polyphosphates to theorthophosphate form. This is particularly true where the aqueous systemcontaining the glassy phosphate is heated to high temperatures. This ismost serious since the corrosion inhibiting power and the calciumsequestering value of the glassy phosphate are lost upon reversion or"glassy phosphates to the orthophosphate form. Additionally, thenon-sequestered hard water ions present in the aqueous system formorthophosphate precipitates which eventually form a scale on theinterior walls of the aqueous carrying conduits. In those applicationswhere the water is being used for heat exchange as in boilers,condensers, and coolers, the orthophosphate precipitate seriouslyinterferes withthe heat transfer of the equipment. As a result, therehas been a serious need for phosphates having both high metal corrosioninhibiting properties in aqueous solution and improved resistance toreversion.

It is an object of the present invention to produce a long-chain sodiumphosphate glass which has high metal corrosion inhibiting properties inaqueous solution and which resists reversion to the orthophosphate form.

These and other obiects will become apparent in the followingdescription of the invention.

it has been found unexpectedly that sodium polyphosphate glasses havingan average mole ratio of mole ratios below about 1.067 do not possessthe desirable corrosion inhibiting properties which the resentpolyphosphate glasses exhibit. One such polyphosphate glass having amole ratio of 1.()2 and a weight percent P 0 of 69.0 permits twentytimes the corrosion obtained when using the present polyphosphateglasses.

The present polyphosphate glasses have the following formulation:

i MO i' o M when n about 20 to 30 and M=at least about mole percent H,with the remainder being Na.

These compounds have been found to have marked superiority overpolyphosphate glasses with higher mole ratios because they do not revertreadily to sodium orthophosphate. These glassy polyphosphates haveparticular application in aqueous systems where the water is heated tohigh temperatures. Examples of such systems are recycle water-cooledcondensers in which the Water removes heat from the condensed liquids,and tubular heaters employing circulating water as the heating medium.

The preparation of the present sodium phosphate glasses is carried outby chemical reaction of a basic inorganic sodium compound and a simpleacidic phosphate. The sodium-containing compound should be one thatprovides a volatile anion such as the hydroxide, carbonate, etc. Theacidic phosphate should provide a volatile cation as do ammonium monoordi-hydrogen phosphate, phosphoric acid, etc. After adjustment of sodiumto phosphorus ratio in the reaction mass, it is heated to temperaturesin excess of about 600 C., resulting in a clear, transparent moltenmass. This molten mass is rapidly chilled by well-known means. Forexample, the molten mass may be poured onto cold surfaces such aswatercooled pans or trays. This solidified mass is crushed or ground toa desired size and packed in air-tight containers.

The weight percent of P in the present polyphosphate glasses varies fromabout 68.5 to 69.7%. At a P 0 weight percent of 68.5% the ratio of z -izis fixed at about 1.10 as the weight percent P 0 increases, the moleratio of decreases so that as a weight percent P 0 of between 69.0 and69.7% the mole ratio of z -lz 2 5 is about 1.067.

The exact weight percent P 0 at any given (Na O+H O) 2 5 ratio in thepresent polyphosphate glasses depends upon the ratio of Na O to H O inthe molecule. The term H O refers to the hydrogen atoms, expressed as HO, which are present at each terminal end of the polyphosphate chain. Asthese hydrogen atoms are replaced with sodium atoms, the weight percentP 0 of the polyphosphate molecule decreases. Since the number ofterminally located sodium atoms which can be present in the instantpolyphosphate molecules is limited to about 25 mole percent of the totalterminal groups, this limits the P 0 weight percent which can be presentin a polyphosphate glass having a given (H 0+Na O) 2 5 mole ratio.

It is desirable to have a very limited number of terminally located ONagroups in order to prevent the glassy polyphosphate from yielding tooacid a solution in those applications where overly acid aqueous mediumsare undesirable. Where the present polyphosphate glasses have onlyterminal OH groups with no ONa groups, aqueous solutions of theseglasses have a pH of about 4.5. These low pH glasses are desirable wherethe system is to be employed under slightly acid conditions. Byreplacing a limited number of terminal OH groups of the polyphosphateglass with ONa groups, the acidity is decreased, making these glassesmore suitable for neutral or slightly alkaline systems. In any event,all the polyphosphate glasses having terminal groups made up of no lessthan about 75 mole percent OH groups with the remainder being ONa groupshave been found to be highly resistant to reversion to sodiumorthophosphate, while still possessing the capacity to inhibit corrosionof metal in contact with the aqueous systems.

An ancillary advantage obtained by decreasing the reversion of theseglasses, is that their calcium sequestering ability remains atrelatively high levels. In contrast, Where severe reversion takes place,as in the prior commercial polyphosphate glasses, the calciumsequestering value falls off sharply and remains at undesirably lowlevels. The present polyphosphate glasses show on the order to three toover four times the sequestering value of commercial glasses after bothhave been exposed to high temperatures for an identical period of time.

The present sodium polyphosphate glasses are employed in water solutionsin amounts of about 1 part per million to about 40 parts per million. Atthese concentrations, the phosphate glasses have been found to beelfective in inhibiting corrosion of metals in contact With the aqueoussystem. Larger amounts can be employed where unusually hard water isencountered.

The sodium polyphosphate glasses can be employed in amounts of fromparts/million to as high as 5% when they are being added to softenextremely hard Water by sequestering metal ions such as magnesium andcalcium. In this application, the metal ions are bound in a solublecomplex rendering them innocuous in the aqueous solution. This softeningtreatment prevents metal ions from interfering in chemical reactionswhen the aqueous solution is used in chemical treating operations, i.e.,bleaching of textiles, forming detergent formulations, etc. Thefollowing examples are presented by way of illustration only and are notdeemed to be limitative of the present process.

EXAMPLE I Phosphorus was burned to P 0 in a Wetted-wall type furnace ofthe type described in US. Patent No. 2,708,- 620 issued to Henry S.Winnicki on May 17, 1955. The P 0 was absorbed in an aqueous solution ofNa HPO Sodium carbonate was added to the solution to adjust the ratio ofmonosodium phosphate to di-sodium phosphate in the solution as required.The mole ratio of NaH PO Na2HPO4 was fixed at 99/1. The density of thatsolution was maintained at 59 Baurn. The solution was maintained at atemperature of about 100 C. during which the reaction Went to completionand CO was liberted. The reaction solution was fed to a furnace at arate of 1170 lbs/hour and heated to a temperature of about 650 C.,during which most of the water was driven off. The resultant clear,transparent molten mass was then quickly chilled to a clear glass whichfractured into relatively small pieces. The particulate glass productwas labeled Glass A, stored in air-tight containers, and permitted tocool.

The weight percent P 0 chain length, and

(Na O|-I-I O) 2 5 ratio are given in Table I.

A second batch of polyphosphate glass was produced in the same manner asdescribed above, except that the P 0 was absorbed in a phosphoric acidsolution. The mole ratio of v H PO was maintained at 58/1 by theaddition of sodium carbonate. The density of the solution fed to thefurnace was 59 Baum. The resultant particulate glass product was labeledGlass B, stored in air-tight containers and permitted to cool. Theweight percent P 0 and (N3.20+H20) P 0 ratio are given in Table I.

5 EXAMPLE n The two glasses made in Example I called Glass A and Glass Bwere compared with atypical commercial product for their calciumsequestering ability. This was done by making up solutions containing 1%of the respective glasses in distilled water, adjusting the pH to 6.5and boiling under reflux at 100 C. for three hours. At the end of thethree hour test, the solutions were tested for calcium sequesteringpower by the method of Hafiord et al., which is described in Ind. Eng.Chem. Anal. Ed. 18, page 411 (1946). The orthophosphate content of thesolutions was also determined by ASTM method D501-58T (1958, Part 10,page 831). Calcium values are expressed as grams of calcium sequesteredper 100 grams of glass.

Table I Mole Percent OH on Terminal Groups Percent Pg (mole ratio) Wt.Percent Reversion (Percent Comm. Class C.

ment over terms of mills per year (m.p.y.). The results of duplicatetests are given in Table III.

Table III (Na O+H O) P205 (Mole Ratio) Wt Corrosion,

In.p.y. percent Sample RunA Blank Before After EXAMPLE III The twoglasses made in Example I called Glass A and Glass B were compared withtypical commercial products for their rates of reversion toorthophosphate. This was done by making up solutions containing 100parts/ million of their respective glass in distilled water, adjustingthe pH to 6.5 and boiling under reflux at 100 C. for three hours. Theorthophosphate content of the solutions were determined analyticallyafter three hours in the same manner as Example II. The results whichwere obtained are given in Table II.

Hot-rolled mild steel test strips were washed with detergent solutions,rinsed dry, degreased with acetone, pickled for one hour in 10%hydrochloric acid, rinsed, brushed with a soft brush, rinded withdistilled water, dipped in acetone, and allowed to air dry. One side ofthe edges of each strip were coated with clear nail enamel and thestrips were then dried in a desiccator and weighed. One liter of tapwater containing 40 parts/million of a sample glass was placed in atwo-liter beaker and the pH of the solution adjusted to 6.0. The teststrips were placed in the beaker exposed side up and the solutionagitated with a four-vaned glass stirrer at 450 r.p.m. At the end ofthree days, the test strips were removed, washed with tap water, rinsedwith distilled water, wiped dry, dried in a desiccator and weighed. Thisprocedure was carried out with solutions of the various samples listedin Table III, as well as a blank tap Water solution containing no addedglass. The loss in weight of the test strips was converted to surfacecorrosion rate The results of Example II as tabulated in Table I clearlyshow the improved resistance to reversion which the present glassespossess.

The percent improvement over commercial glass, as reported in Table I,indicates 28 to 31% less orthophosphate is produced by the presentglasses compared with commercial glasses. cium sequestering values,given in Table I, show that the The calcommercial glasses initially havesomewhat equal sequestering power as the present glasses.

However, upon treatment at higher temperatures which acceleratereversion, the present polyphosphate glasses have 3 to 4% times thecalcium sequestering power of the commercial preparation.

The results of Example IV, as reported in Table III,

demonstrate the corrosion inhibiting power of the present glasses. ousincrease in corrosion which is obtained with poly- Additionally, Table1H points out the seriphosphate glasses having mole ratios of below1.067.

(Na O+I-I O) P 0 In this case, Comm. Glass C, Glass A,

and Glass B all had corrosion rates of about the same order ofmagnitude, whereas, commercial Glass X, with mole ratio of z -i z P 0 of1.029, showed corrosion rates twenty times as great. Such corrosionrates are beyond acceptable limits and show lack of material corrosioninhibiting properties by commercial Glass X.

Pursuant to the requirements of the patent statutes,

cluding what is considered to be the best embodiment of the invention.

However, it should be clearly understood that, within the scope of theappended claim, the invention may be practiced by those skilled in theart, and having the benefit of this disclosure, otherwise than asspecifically described and exemplified herein.

What is claimed is: The method of softening hard water by sequesteringmetal ions which comprises adding to said water from about ppm. to about5% by weight of a sodium phosphate glass having improved resistanceagainst reverchain having no less than 75 mole percent OH with theremainder being ONa groups.

References Cited in the file of this patent 5 Phosphates in WaterConditioning, Schwartz et a1., Ind. and Eng. Chem, January 1942, vol.34, pp. 3240.

